1. Field of the Invention
This invention relates to a receiver system in a mobile communication base station which receives signals of such a mobile communication as an Analog Cellular, a Digital Cellular, a Code Division Multiple Access (CDMA) Cellular, a Personal Communication Service (PCS), a Trunked Radio System (TRS), a Dual Paging, and a Wireless Local Loop (WLL) and more particularly, to an outdoor receiver system of a mobile communication base station which prevents the deterioration of the characteristics of the receiver system due to a variation of external environment by establishing the receiver system of the base station outdoors and keeping its temperature constant all the time by use of a thermal electric module so that a maximal communication distance for up-link may be increased.
2. Description of the Related Art
Typically, in such a mobile communication service as an Analog Cellular, a Code Division Multiple Access Cellular (CDMA), a Personal Communication Service (PCS), a Trunked Radio System (TRS), a Dual Paging, a Wireless Local Loop (WLL), a transmitter system in the base station transmits large output signals by using a power amplifier with large output so that a portable terminal in a mobile station at a long distance from the base station can receive the signals. In the portable terminal in the mobile station, however, due to the constraint on capacity of a battery and size of the portable terminal, a large power output is impossible but an output with only an order of several hundred milliwatts to several watts can be generated. As a result, a distance at which signals transmitted from the portable terminal in the mobile station can be received in the receiver system in the base station become relatively very short.
For a mobile communication service, the difference in a communication distance between a down-link from the base station to the mobile station and an up-link from the mobile station to the base station requires that more base stations should be established, resulting in the increase of the costs of establishment, maintenance and repair.
Under such the circumstances, for the purpose of reducing the difference in the communication distance between the down-link and the up-link, studies have put emphasis upon less noise figure of a Low-Noise Amplifier (LNA) and less insertion loss of a BandPass Filter (BPF) or Duplexer Filter to improve reception sensitivity of the receiver system in the base station.
However, as shown in FIG. 1, an existing receiver system in the base station has a bandpass filter, a low noise amplifier, and a down converter, etc., located indoors, a coaxial cable of 10 .about.several ten meters with insertion loss in order of several dBs is connected between an antenna located at a tower and the receiver system of the base station. Accordingly, the insertion loss of the coaxial cable is reflected, as it stands, in a total noise figure of the receiver system, resulting in the increase of noise figure by the insertion loss of the coaxial cable.
The total reception noise figure in the receiver system connected as shown in FIG. 1 can be expressed as follow: EQU NF.sub.total .apprxeq.L.sub.c +L.sub.b +NF.sub.1
Where, Lc indicates the insertion loss of the coaxial cable, Lb indicates an insertion loss of the bandpass filter, and NF1 indicates a noise figure of the low noise amplifier.
For example, in the case where the insertion loss of the coaxial cable Lc.apprxeq.4 dB, the insertion loss of the bandpass filter Lb.apprxeq.1 dB, the noise figure of the low noise amplifier NF1.apprxeq.1 dB, and a gain of the low noise amplifier G1.apprxeq.40 dB, then the total noise figure NF.sub.total of the receiver system is obtained as follows by using the above expression. EQU NF.sub.total .apprxeq.4+1+1.apprxeq.6 dB.
At this time, as the gain of the low noise amplifier is very large as 40dB, noise figures in the down-converter and the followings make little contribution to the total reception noise figure of the receiver system.
Accordingly, with only the method by which the insertion loss of the bandpass filter is reduced or the noise figure of the low noise amplifier is decreased in the receiver system connected as shown in FIG. 1, the total reception noise figure of the receiver system in the base station can not be lowered any more, and then the increase of the maximal communication distance for the up-link by the improvement of the reception sensitivity can not be expected any more.
A method for overcoming such a shortcoming is to establish the bandpass filter and low noise amplifier (BPF+LNA) or the bandpass filter, low noise amplifier, and down-converter (BPF+LNA+D/C) (not shown) at a location (outdoors) as close as possible to the receiving antenna as shown in FIG. 2 so that the noise figure of the receiver system by the insertion loss of the coaxial cable may be reduced. This has been known to be the only method to improve significantly the reception sensitivity of the receiver system in the base station.
Under such circumstances, similar to the case described with respect to FIG. 1, in the case where the insertion loss of the coaxial cable Lc.apprxeq.4 dB, the insertion loss of the bandpass filter Lb.apprxeq.1 dB, the noise figure of the low noise amplifier NF1.apprxeq.1 dB, the gain of the low noise amplifier G1.apprxeq.40 dB, then the total reception noise figure (NF' total) of the receiver system is obtained as follows. EQU NF'.sub.total .apprxeq.L.sub.b +NF.sub.1 =1+1=2 dB
From the above expression, it can be seen that the total reception noise figure of the receiver system is improved or lowered by the amount of insertion loss of the coaxial cable (.apprxeq.4 dB) because the coaxial cable is not included in the receiver system any more.
Although the idea on the improvement of the reception sensitivity with the consequence of the extension of the maximal communication distance for the up-link is well-known to those skilled in the art, the receiver system of the mobile communication base station lies in real situation of being established indoors other than outdoors because of the difficulty of implementing the idea in reality.
In addition, when the receiver system is established outdoors, however, there is a problem that the characteristics of the receiver system is changed with the change of external environment.
More specifically, firstly, as temperature is changed, the characteristics of the bandpass filter (or duplexer filter) is changed. In other words, if the receiver system is established outdoors, the temperature environment of the receiver system is changed in the range from about -30.degree. C. to about 80.degree. C. to thereby change the frequency characteristics of the bandpass filter by the order of several MHz resulting in the occurrence of communication jamming.
Also, the characteristics of the low noise amplifier (or including down-converter) is significantly changed with the rising of temperature. Consequently, the gain of the low noise amplifier grows reduced and the noise figure of the same grows increased.
In order to prevent the deterioration of characteristics of the outdoor receiver system caused by such a temperature change, the outdoor receiver system should be placed in a thermostat.
In keeping the temperature of the thermostat constant, a low ambient temperature does not matter particularly because the temperature of the thermostat can be raised by a heater provided therein. However, if an ambient temperature is high, an air conditioner should be provided within the thermostat. However, considering the existing state of the art, it is not easy to establish, maintain and repair the outdoor receiver system in which the air conditioner is employed because of an excess of volume and weight of, for example, a condenser fan as a part of the air conditioner.
The maintenance and repair include the periodic supplement of a refrigerant and the settlement of mechanical trouble of such rotating portion as the condenser fan. As a result, weak durability and mechanical shaking of the air conditioner are apt to have a bad influence upon the characteristics of the receiver system.
On the other hand, the receiver system has been known which keeps the temperature of the bandpass filter made into high temperature superconducting material constant to about 77.degree. K. in absolute temperature by using liquid nitrogen to prevent the deterioration of the characteristics of the outdoor receiver system caused by the change of temperature. However, since a compressor should be employed in this receiver system to provide liquid nitrogen, this receiver system has no practical use owing to the difficulty of establishing, maintaining and repairing the compressor as well as the cost of it.
Therefore, there is a need for an improved thermostat which has light weight and small volume, facilitates the maintenance and repair, and has excellent durability and high reliability in order to prevent the deterioration of characteristics from generating when the receiver system in the mobile communication base station is established outdoors.